A Mn—Mo—Ni steel material has excellent strength and toughness, and is used mainly in a pressure vessel of an atomic power plant, or the like. The pressure vessels have become larger in recent years with an increase in energy demand, which needs Mn—Mo—Ni steel materials having more excellent strength and toughness. Together with this, a Mn—Mo—Ni based weld metal formed in a weld joint of a welded structure using such a type of steel as the base material is also required to have improved strength and toughness.
Welded structures using the Mn—Mo—Ni steel material as the base material are generally subjected to stress relief heat treatment for a long time after the welding so as to relieve the stress. The heat treatment often degrades the strength and toughness of the weld metal in the welded structure. Thus, the weld material whose strength and toughness is less likely to deteriorate due to the heat treatment is required.
For this requirement, various techniques are proposed for improving the strength and toughness of the weld metal (hereinafter totally referred to as “mechanical characteristics”). For example, Patent Document 1 proposes a technique for improving the mechanical characteristics by appropriately controlling the composition of alloy elements, such as Cr, Mo, Cu, Ti, or B. Patent Document 2 proposes a technique for improving the mechanical characteristics of a flux cored weld wire after stress relief heat treatment by simultaneously controlling the compositions of a metallic shell and a flux. From the view point of control of a sulfide, Patent Document 3 proposes a weld material having excellent weldability, in addition to the adequate mechanical characteristics. Patent Document 4 proposes the control of a carbide for the purpose of improvement of the mechanical characteristics after heat treatment.